In optical coherence tomography (OCT) imaging, effort is placed on obtaining high image quality to allow for reproducible and clear visualization of structures and pathologies as well as quantitative measurements of features and layers within the eye. Typically OCT measurements of the posterior section of the eye are made with the focus of the beam in the plane of the pupil and the beam entering through the center of the pupil. In theory this allows the largest possible entry and exit pupil, allowing for optimal collection of the OCT signal as well as any additional signals used for alignment purposes. The resulting retinal image shows bands of varying reflectivity signals that have been correlated to layers identified in histology. Segmentation of the retinal tissue is typically made based on the observed reflectivity differences between layers, although information about the expected configuration of the layers may also be used. It has recently been recognized that the reflectivity of some structures in the eye may depend on the local tilt of the retina relative to the OCT beam.
Although a central entry point is nominally optimal, there are a number of reasons to use entry points that are not central. In subjects with media opacities such as cataracts, the measurement beam may not pass well through the opacity. In such cases, it is sometimes possible to steer the measurement beam through a different entry position so that the opacity is avoided. In other subjects, the shape of the eye may be such that the image of the retinal tissue appears tilted. A different entry point in the pupil may result in a flatter image. Because layer measurements are typically made along A-scans, a flatter retina may result in measurements with less geometrical error. Furthermore, since many OCT systems have decreasing signal quality further from the zero delay, a flatter retina may have better uniformity of intensity across the B-scan. Finally, some tissues in the eye have reflectivity that depends on the angle of incidence. Ensuring a flat retina on each visit reduces the variance of incidence angle over multiple visits, which reduces the impact of directional reflectivity on the variability of measurements made on the image. Alternately, optimal imaging of tissues with strong directional reflectivity may require a specific angle of incidence which by geometry requires a different pupil entry location, or may even require scans with multiple angles of incidence (and therefore multiple pupil entry locations) to be combined prior to layer detection.
In current systems, the user has to manually adjust the pupil entry position and find the “best” entry position for the particular subject and imaging application by a trial and error method. This is a subjective procedure, in which the operator has to review the OCT scan, the fundus image, and the iris image in order to determine the alignment that results in an optimal compromise between OCT signal quality, B-scan tilt, and fundus image quality. Though it is not current practice, in the future users may also wish to optimize based on the specific reflectivity profile of given layers.
Once the best position is identified, it is still difficult to maintain the entry position for the duration of the scan because of eye motions or changes in gaze. This effect is particularly important in OCT systems where the scans usually take a few seconds and a dense cube of data is acquired.
Various attempts have been made to increase feedback to the operator and automate aspects of data acquisition to achieve the highest quality images possible. Retinal tracking systems have been described (see for example US Patent Publication No. 2005/0024586, U.S. Pat. No. 7,480,396 and U.S. Pat. No. 7,805,009, and U.S. patent application Ser. No. 13/433,127, filed Mar. 28, 2012, hereby incorporated by reference) to compensate for motion of the retina during retinal imaging. However, retinal tracking methods usually work by analyzing images of the back of the eye that are mostly obtained by point scanning or line scanning devices that also depend on an optical path that goes through the pupil. If the scan entry position is not optimal, the images used for tracking the retina will also be affected, resulting in poor quality of tracking or a total failure to track if the fundus image quality degrades significantly.
It is therefore an object of the current invention to address some of the limitations described above. In particular, it is an object of the present invention to automate the process of finding the best position to take OCT measurements as well as maintaining that position over the acquisition time to ensure that the optimal signal is obtained and maintained, allowing measurements to be taken automatically and without user intervention. This invention further makes it possible to have a set of scan patterns optimized for different structures in the eye and allows the system to automatically place the beam at the optimal angle for each scan pattern. The information from the scans taken at multiple locations through the pupil can also be combined to produce a comprehensive view of the eye. The invention further makes it possible to ensure that a scan acquired on a future visit is acquired with the same pupil entry position, reducing any effect that variability of pupil entry location has on the variability of image quality or on variability of quantitative measurements.